One known rotation transmission device used to selectively allow and prohibit transmission of rotation includes a two-way clutch configured to selectively couple and uncouple an input shaft and an output shaft, and an electromagnetic clutch configured to selectively engage and disengage the two-way clutch.
A rotation transmission device disclosed in JP Patent Publication 2009-293679A includes a control retainer and a rotary retainer that are mounted between an outer ring provided at an end of an output shaft and an inner ring provided at an input shaft so as to be disposed inside of the outer ring such that pillars of the control retainer circumferentially alternate with pillars of the rotary retainer, with pockets defined between adjacent pairs of the pillars of the respective retainers. The rotation transmission device further includes opposed pairs of rollers, each pair being received in one of the pockets, and elastic members each mounted between each opposed pair of rollers to bias the pair of rollers away from each other to standby positions where the rollers are engageable with a cylindrical surface formed on the inner periphery of the outer ring and cam surfaces formed on the outer periphery of the inner ring. When the inner ring rotates in one direction, one of each opposed pair of rollers engage the cylindrical surface and the cam surface, thereby transmitting the rotation of the inner ring to the outer ring.
This rotation transmission device further includes an electromagnetic clutch provided on the input shaft, which is provided with the inner ring, and is configured such that when the control retainer is moved by the electromagnetic clutch in one axial direction, the control retainer and the rotary retainer are rotated relative to each other in the direction in which the circumferential widths of the pockets decrease, by torque cams provided between opposed surfaces of flanges of the control retainer and the rotary retainer. As a result, each pair of rollers is moved, respectively, by the pillar of the control retainer and the pillar of the rotary retainer to disengaged positions, so that rotation is not transmitted from the inner ring to the outer ring.
This rotation transmission device is configured such that when the electromagnetic coil of the electromagnetic clutch is de-energized, the control retainer is moved in the direction in which the flange of the control retainer moves away from the flange of the rotary retainer. Simultaneously, the control retainer and the rotary retainer are rotated relative to each other in the direction in which the circumferential widths of the pockets increase under the biasing force of the elastic members mounted between the respective opposed pairs of rollers, so that the opposed pairs of rollers instantly engage the cylindrical surface and the cam surfaces. This rotation transmission device is thus advantageous in that there exist little gaps in the rotational direction, so that the rotation of the inner ring can be instantly transmitted to the outer ring.
In the rotation transmission device disclosed in JP Patent Publication 2009-293679A, if attempts were made to increase the hardness of the control retainer and the rotary retainer by heat-treating the entire retainers, their toughness would decrease to such an extent that cracks tend to develop at the roots of the pillars due to stress concentrations at the roots of the pillars. Thus, the control retainer and the rotary retainer of this rotation transmission device are used without being heat-treated.
In this case, since inclined cam surfaces of cam grooves forming each torque cam, on which a ball of the torque cam rolls, are low in hardness, the inclined cam surfaces tend to become worn due to contact with the ball. Also, the portions of the inclined cam surfaces that are brought into contact with the ball tend to develop dents. When the ball falls into such dents, the ball will interfere with relative rotation between the control retainer and the rotary retainer, thus affecting smooth operation of the two-way clutch. It is therefore desired to further improve the durability of the retainers of this rotation transmission device.